The mode of action of complement on biological membranes is poorly understood. A unique approach to the study of the action of complement is proposed here in which mutant strains of a Mycoplasma that are resistant to the immune lysis are used to study the chemical nature of membrane components to which complement proteins bind and how the binding of these proteins affects the molecular structure of cell membranes. The research entails studies that compare the chemical composition of the membranes of the normal and the mutant strains, the membrane topography of normal and mutant strains, and the binding of complement proteins to the membranes of the mutant strains. The methods used include screening for mutant strains, lysis of cells by antibody and complement, SDS-polyacylamide gel electrophoresis of isotopically labeled membrane polypeptides, thin layer chromatography, gas chromatography, locating membrane proteins with isotopically labeled reagents, and freeze-cleave electron microscopy.